This invention relates to transfer printing, and more particularly to electrostatic transfer printing.
Various types of electrostatic transfer printers can be found in the prior art. Examples are F. A. Schwertz U.S. Pat. No. 3,023,731; Richmond Perley U.S. Pat. No. 3,701,996; and T. Doi et al, U.S. Pat. No. Re. 28,693. Electrostatic transfer printers may be classified generally according to the way in which the latent electrostatic image is formed. One prior art approach utilizes metal styli at minute distances from the surface of the dielectric transfer drum. The styli are electrically pulsed to provide a latent electrostatic image by air gap breakdown. This technique has the disadvantage of not allowing for multiplexing of the charging styli. In addition, the necessity for maintaining a very small air gap breakdown distance requires extremely close tolerances which limit the practicability of this technique.
Another type of electrostatic printer found in the prior art employs an ion source in the form of a corona point or wire used together with an image defining mask. Because of the inherently low current densities available from corona discharges, this method is impractical for high speed printing. The use of coronas also poses significant difficulties in maintenance. Corona wires are fragile, and because of their high operating potentials, tend to collect dirt and dust. Hence they must be frequently cleaned or replaced.
Another approach to electrostatic transfer printing depends on the method by which the toned image is transferred and fused onto the receptive sheet. The transfer printing system, R. Perley U.S. Pat. No. 3,701,996, teaches simultaneous transfer and pressure fusing by passing a receptive sheet between the transfer and pressure drums. In P. Pederson, U.S. Pat. No. 3,874,894, a nylon-6 sleeve is provided on at least one of a pair of pressure rolls, but the drums are used only for fixing the already transferred toner, an arrangement which adds significant complexity to the overall system.
Accordingly, it is an object of the invention to facilitate electrostatic transfer printing. A related object is to reduce critical mechanical tolerances in providing a latent electrostatic image. Another related object is to reduce the maintenance problems associated with the formation of such an image.
A further object of the invention is to achieve increased electrostatic printing speed. A related object is to do so by using a reliable, easily controlled ion source. A still further object is to achieve relatively uniform charge images which may be toned with good definition and dot fill. A further related object is to provide a matrix selection (or multiplexed) method of dot matrix printing.
Another object of the invention is to achieve a transfer drum with surface resistivity sufficient to prevent image degradation from the time when the image is presented to the surface until the image is toned. Still another object is to utilize a surface with high abrasion resistance, and sufficient smoothness to provide complete transfer of toner to a receptor sheet. A still further object is to realize a transfer surface not subject to significant distortion.
Yet another object is to facilitate the erasure of latent residual electrostatic images. A related object is to avoid ghost images in subsequent printing cycles.